That big yellow block hovering in the sky!
The Sun is a sprite appearing only in Indev and Infdev that lights up all blocks with nothing above them while it is in the sky. When the player creates a new map the sun starts on the horizon at dawn. As time passes, it will slowly move across the sky and over the horizon while the moon rises on the opposing horizon. Daytime lasts 11 minutes and grass blocks will only spread when exposed to sunlight.
Sunlight
Sunlight, in game, is the name given to the light which illuminates all blocks which do not have shadow cast over them by a non-transparent block positioned vertically above them in the sky.
Sunlight is one of the two requisites to turn dirt blocks into grass blocks (the other being that there must be another sun-affected grass block adjacent to it). Sunlight alters the brightness of the blocks it falls upon. Patterns suspended in the air for sunlight to shine through affect the colours on the ground. This can be used to provide more shades of colour for more compliacted patterns on floors and such.